Tools for cutting and/or abrading are conventionally fabricated of a suitable matrix material with minute abrasive grains, such as diamonds, embedded within the matrix material. Basically, such tools are formed by conventional powder metallurgical techniques, wherein the abrasive grains are initially mixed with the matrix material (e.g. metals, alloys, metal carbides etc., as well as mixtures thereof) in powder form and some binding agent, after which the mixture is press-moulded to bond and bring the mixture into the desired tool shape. In the so-called hot-pressing method, the mixture is placed in a mould having the shape of the abrasive tool to be formed and pressed at high pressure and temperature to cause sintering of the sinterable material. According to the “cold press and sinter” technique, the mixture is first pressed at high pressure into the desired tool shape and thereafter fired at high temperature in a furnace to sinter the tool. As an alternative to these compaction techniques, it is known, for instance from document EP 0 754 106 B1, to provide soft and easily deformable preforms in the form of a slurry or paste containing the matrix material in powder form, abrasive grains and some liquid binder phase. The soft and easily deformable preforms are subsequently sintered and/or infiltrated. Tools fabricated in these or similar manners are commonly referred to as metal-bonded abrading or cutting tools.
Efficiency and lifetime of an abrading or cutting tool are among others determined by the degree of uniformity of the distribution of the abrasive grains on the surface or volume of the tool and by the retention strength of the abrasive grains within the surrounding matrix material. In tools fabricated according to any of the above-described techniques, the abrasive grains are randomly distributed, which means that some of them may be close to each other, possibly touching each other, while some regions of the tool may only have little density of abrasive grains. As a matter of fact, this negatively affects the cutting or abrading performance of the tool.
An important step towards uniform distribution of abrasive particles throughout the matrix material was the method taught in U.S. Pat. No. 3,779,726. This document proposes to tumble the abrasive grains in the presence of a powder of sinterable material and binding agent while controlled amounts of water are simultaneously sprayed thereon. In this way, each abrasive particle is singularly coated with a sinterable particulate mass in such a way that granules, so-called “pellets”, are formed. These pellets are subsequently pressed into the desired shape at high pressure (35 tons per square inch, i.e. approximately 500 MPa), possibly after being mixed with granulated metal powder. Those skilled in the art are aware that mixing the pellets with metal powder may cause the problem of segregation between the metal powder and the pellets. Another method for individually coating abrasive grains is disclosed in U.S. Pat. No. 4,770,907. This method commences with the preparation of a slurry of a selected metal powder with a binding agent dissolved in an organic solvent in predetermined relative concentrations. The abrasive grain cores are then fluidized in a work vessel and the slurry is sprayed onto the abrasive grains cores during fluidization, whereby a generally uniform coating of the slurry builds and dries on each abrasive grain. A further improvement in terms of particle distribution has been reached with the method of WO 2008/025838 A1, according to which a soft, easily deformable paste is formed which has superabrasive particles dispersed therein, each of these particles being individually encrusted within a coating of presintered material. The presintered coating keeps the superabrasive particles at least at a distance of twice the thickness of the coatings from one another. These coatings also prevent a direct contact between the superabrasives and the mould or the walls of an extrusion/injection moulding system.
US patent application 2008/0017421 A1 discloses encapsulated particles, whose encapsulation (“shell”) comprises sinterable matrix material and abrasive particles. The composition of the matrix as well as type, size and density of the abrasive particles in the shell can be selected depending on the specifications for the abrading or cutting tool. To produce individually encapsulated particles having similar amounts of shell material and approximately the same size, US 2008/0017421 A1 suggests mixing the particles to be encapsulated, matrix powder and the abrasive particles for the shell in a conventional mixing machine and processing the resulting mix with a granulator, in which the mix is extruded into short “sausage” shapes, which are then rolled into balls (granules) and dried. As an alternative, the document proposes the using the so-called Fuji-Paudal pelletizing machine, disclosed e.g. in U.S. Pat. No. 4,770,907. As regards the “sausage” method, the granules that are formed are not uniform so that a step of selecting granules of substantially the same size is requires. This method has the further drawbacks that each granule formed does not necessarily contain one (and only one) core particle and that the core particle is not necessarily positioned centrally within its shell. Thus 2008/0017421 A1 does not enable the production of pellets having a single abrasive particle in the centre.
Neither U.S. Pat. No. 4,770,907 nor U.S. Pat. No. 3,779,726 disclose methods allowing coating core particles with shell particles whose size approaches the size of the core particles: the ratio of the average diameter of the shell particles to the average diameter of the core particles should not exceed 1/7. Nevertheless, one already encounters serious problems with the above methods as the ratio exceeds 1/11. Another problem arises if the abrasive particles to be coated and the particles used as the surrounding material exhibit very different densities (e.g. diamond: 3.5 g/cm3, coarse WC: 15.8 g/cm3, fused WC: 16.4 g/cm3) because the particles tend to segregate. The difficulties caused by density differences are particularly pronounced in the Fuji-Paudal method. The quality of the results achieved furthermore depends on the shape of the particles involved.
International patent application WO 2009/118381 proposes a method and an apparatus for producing aggregate abrasive grains, wherein the diameter of the peripheral particles can be closer to the diameter of the core particles. According to this document, abrasive core particles are coated with adhesive, the adhesive comprising a binding agent and a solvent therefore. The adhesive-coated core particles are dropped onto a layer of abrasive particles (which are to become the peripheral particles of the aggregate abrasive grains) and covered with further peripheral particles. The aggregates so obtained are then consolidated by causing the solvent to evaporate, i.e. by letting the adhesive set. WO 2009/118381 further discloses aggregate particles with a central diamond surrounded by a plurality of smaller peripheral diamonds, which are attached to the core diamond with diamond-diamond bonds.